1. Field of the Invention
The present invention relates to surface acoustic wave (hereinafter referred to as “SAW”) devices used as filters which are suitable for narrow-band communication apparatuses. More particularly, the present invention relates to a SAW device using a plurality of ladder-type one-terminal-pair SAW resonators and also to a communication apparatus including such a SAW device.
2. Description of the Related Art
An example of a known SAW device used as a bandpass filter which is suitable for a small communication terminal, such as a cellular telephone, is disclosed in Japanese Unexamined Patent Application Publication No. 5-183380. In this publication, a ladder-type bandpass filter including one-terminal-pair SAW resonators alternately connected to series arms and parallel arms is provided.
In the ladder-type bandpass filter disclosed in the above-mentioned publication, as shown in FIG. 15, first one-terminal-pair SAW resonators 51a, 51b, and 51c are connected in series with each other, and second one-terminal-pair SAW resonators 52a, 52b, 52c, and 52d are connected in parallel with each other. Then, as shown in FIG. 16, the anti-resonant frequency (fap) of the parallel resonators 52a, 52b, 52c, and 52d is allowed to substantially coincide with the resonant frequency (frs) of the series resonators 51a, 51b, and 51c. The resulting ladder-type filter exhibits very good characteristics, for example, low loss and a wider band, and is thus widely used, mainly, as a communication filter.
The above-described publication also discloses that a wider bandwidth can be obtained by setting the resonant frequency (frs) of the series resonators 51a, 51b, and 51c to be higher than the anti-resonant frequency (fap) of the parallel resonators 52a, 52b, 52c, and 52d. 
However, wider bandwidth filter characteristics are not always suitable, and the bandwidth should be determined according to the specifications of the filter.
As shown in FIGS. 17 and 18, filter characteristics having narrow bandwidths are necessary for Global Positioning System (GPS) or Time Division Synchronous Code Division Multiple Access (TDS-CDMA) filters, respectively. Personal Handyphone System (PHS) or Personal Digital Cellular (PDC) filters also require narrow bandwidths. The pass bands of GPS, TDS-CDMA, PHS, and PDC filters can be indicated in terms of the bandwidth ratio as 0.2%, 0.7%, 1.8%, and 1.7%, respectively. The bandwidth ratio is the ratio determined by dividing the pass bandwidth by the corresponding central frequency. The bandwidth ratios of other existing filter specifications such as PCS, Digital Command Signal (DCS), and Wideband CDMA (WCDMA) filters are 3.1%, 4.1%, and 2.8%, respectively.
Wider bands are demanded for the filters of the second group, and thus, the design type disclosed in the above-described publication is desirable for such filters. However, for the filters of the first group, narrower bands are demanded, and thus, a known design type, such as that disclosed in the above-described publication, is not desirable. At present, there is no suitable design type for filters of the first group, and more specifically, for filters having a bandwidth ratio of 2.5% or lower.
A narrower bandwidth in accordance with the corresponding specification can be adjusted to a certain degree by changing the type of piezoelectric substrate or decreasing the thickness of a piezoelectric substrate. However, in the above-described known design type, there are limitations to making such adjustments. In particular, for high frequency filters having a central frequency higher than 1.5 GHz, the substrate is already thin because of the high frequency, and a further decrease in the thickness of the substrate in order to obtain narrow-band characteristics results in an increased ohmic loss. It is thus necessary that narrow-band characteristics be obtained without making the substrate very thin.
Thus, according to the above-described design type, a narrower bandwidth increases the insertion loss. More specifically, the above publication discloses that a wider bandwidth can be obtained by setting the resonant frequency (frs) of the series resonators 51a, 51b, and 51c to be higher than the anti-resonant frequency (fap) of the parallel resonators 52a, 52b, 52c, and 52d. Conversely, as shown in FIG. 19, it can be considered that a narrower bandwidth is obtained if the resonant frequency (frs) of the series resonators 51a, 51b, and 51c is set to be lower than the anti-resonant frequency (fap) of the parallel resonators 52a, 52b, 52c, and 52d. However, an ideal narrow bandwidth cannot be obtained.
FIG. 20 illustrates comparison results of transmission characteristics when the anti-resonant frequency is substantially equal to the resonant frequency, i.e., fap≈frs, and when the anti-resonant frequency is greater than the resonant frequency, i.e., fap>frs. FIG. 20 shows that the insertion loss of the entire pass band is considerably increased when fap>frs. Accordingly, even though the resonant frequency (frs) of the series resonators 51a, 51b, and 51c is set to be lower than the anti-resonant frequency (fap) of the parallel resonators 52a, 52b, 52c, and 52d, it is difficult to decrease the pass bandwidth without increasing the insertion loss.
One way to solve this problem is disclosed in Japanese Unexamined Patent Application Publication No. 11-163664. The number of electrode fingers is reduced so as to decrease the frequency interval between the resonant frequency and the anti-resonant frequency, thereby making the bandwidth narrower.
However, a smaller frequency interval between the resonant frequency and the anti-resonant frequency decreases the impedance at the anti-resonant frequency so as to reduce the Q factor. Thus, the pass bandwidth can be decreased, but on the other hand, the insertion loss is increased.
Japanese Unexamined Patent Application Publication No. 10-13187 discloses a technique for suppressing increased insertion loss in the lower frequency range of the pass band by providing a capacitive device or an inductive device for inhibiting mismatch loss between the stages.
However, this is not a suitable technique for decreasing the pass bandwidth. Thus, the configuration and the concept disclosed in that publication are different from those of the present invention.
Japanese Unexamined Patent Application Publication No. 2002-232264 discloses a technique for increasing the attenuation in the lower frequency range of the pass band by setting the anti-resonant frequency of one series resonator to be lower than the resonant frequency of parallel resonators.
However, this is not a suitable technique for decreasing the pass bandwidth. This is merely a technique for increasing the attenuation in the lower frequency range of the pass band by setting the anti-resonant frequency of parallel resonators to be equal to the resonant frequency of series resonators, unlike the present invention in which the resonant frequency of the series resonators is set to be lower than the anti-resonant frequency of the parallel resonators. Thus, the configuration and the concept disclosed in that publication are different from those of the present invention.
Japanese Unexamined Patent Application Publication No. 11-312951 discloses the following technique. A plurality of SAW resonators including at least one resonator whose resonant frequency is different from that of the other resonators are connected in series with a series arm. A plurality of SAW resonators including at least one resonator whose resonant frequency is different from that of the other resonators are connected in parallel with a parallel arm. With this configuration, the pass band is decreased.
According to the technique disclosed in this publication, however, the difference Δf between the resonant frequency and the anti-resonant frequency is made smaller by setting the anti-resonance point of the parallel resonators to be substantially equal to the resonance point of the series resonators, thereby decreasing the pass bandwidth. Thus, bottom characteristics are deteriorated, or large ripples occur in the vicinity of the pass band.